Assuming your blocks vary in size no more than 1/2", the following should work.
Sew one horizontal row together. Sew the second horizontal row together. Pin the two rows together at each seam. If you get to one that is hugely variant, you can go to the sewing machine and take in a seam simply by sewing a larger seam right next to the seam that is too narrow (you do not have to take the old seam out when you do this). Once you have the two rows pinned together, sew the seam and ease or stretch to make sure the pins match. You probably do not want to have to ease in more than 1/4" per block.
A photo would help. Also measuring your blocks and giving us the smallest and largest measurement would help.